disambiguating type family instances with qualified names not possible

While reading 1, it occured to me that type families could be used to parameterize modules by types. So I modified my example from 2, trying to parameterize two modules A and B with a shared type Label (sharing expressed in C), but ran into a few issues:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE EmptyDataDecls #-}
module LA where 
data MyLabel
y = undefined::MyLabel

type family Label a
z = undefined::Label ()

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE EmptyDataDecls #-}
module LB where 
data MyLabel
y = undefined::MyLabel

type family Label a
z = undefined::Label ()

{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE TypeFamilies #-}
module LC where
import LA
import LB

-- fails = [LA.y,LB.y]

-- express type sharing while leaving actual type open
type family Label a
type instance LA.Label a = LC.Label a
type instance LB.Label a = LC.Label a
ok2 = [LA.z,LB.z]

-- for testing only
-- type instance Label a = () -- can't use with or without qualifier:-(

Issues:

• is it really necessary for type families to have at least one index? Without that, type constants could be expressed directly

• uncommenting that last line demonstrates a couple of bugs:

• as it stands, the type instance is ambiguous, but the error message has an incorrect source location (1:0)

• trying to disambiguate by defining type instance LC.Label results in : "Qualified name in binding position: LC.Label" (note that this is permitted a couple of lines up, so it is related to whether the qualifier refers to the current module or an imported one)

[the bug is not really in the type checker, but specific to type families..]

Bug aside, it works (length ok2 gives two, not an error, as fail would).

This could be a useful type family programming pattern (it is probably implicit in the comparisons of SML functors vs Haskell type classes/families, I just don't recall it being made so explicit before, just focussing on type parameterization and sharing)!-)

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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